Micro gyros are angular velocity sensors used widely in the automotive, aerospace, consumer, and industrial applications. Unlike conventional mechanical and optical gyroscopes which are relatively large and are fabricated using complex and precision assembly, micro gyros are small and batch-produced using semiconductor wafer fabrication process. One processing technology commonly used for fabrication of the micro gyros is the Micro-Electro-Mechanical Systems (MEMS) technology. Micro gyros made using MEMS are very small with the smallest features measuring only a few micrometers long. With very small features and precise processing control, MEMS micro gyros are highly reliable and robust.
Processes for manufacturing MEMS micro gyro devices are well documented in the literature. Examples of MEMS devices are disclosed in U.S. Pat. No. 6,089,089, “Multi-Element Micro Gyro”, to Hsu; and U.S. Pat. No. 6,578,420, “Multi-Axis Micro-Gyro Structure”, to Hsu, and the entirety of each of which is fully incorporated by reference herein.
The operating principles of vibratory micro gyros are well understood and widely reported in the literature. Generally, a small mass (often referred to as the proof mass resonator) is supported by springs (or flexures) and suspended over a substrate. Through a range of actuation forces, such as electrostatic, thermal or magnetic force, the proof mass resonator is driven to oscillate linearly or rotationally about a first axis (the Drive axis). When the substrate, along with the oscillating mass, is rotated at an angular rate about a second axis (the Rate axis), a virtual force is produced about the third axis (the Sense axis). The three referenced axis (Drive, Rate, and Sense axes) are mutually orthogonal to each other. The resulting output force, known as the Coriolis force, is proportional to the product of the linear velocity of the oscillating mass and the externally applied angular velocity. The Coriolis force acts on the proof mass resonator about the Sense axis, resulting in a small displacement that can be measured using special detectors. The methods of measuring the displacement of the proof mass resonator vary depending on the individual design; commonly used techniques include measuring changes in electrical capacitance, electrical resistance, piezoelectric voltage, piezoelectric resistivity, magnetic field intensity, and optical intensity.
Among the detection techniques mentioned above, measuring the displacement by electrical capacitance is commonly used. With proper compensation circuits, capacitance measurements can be highly accurate and stable over a large temperature range. For very low angular rates, the magnitude of the Coriolis force produced by the proof-mass can be very small (pico-Newtons), hence the displacements of the proof mass resonator are also extremely small (pico-meters). To measure such small displacements, a capacitor is formed by using the proof mass resonator as a moveable electrode and by placing a stationary electrode positioned at a small offset distance from the proof mass resonator. A variety of electronic circuits are used to convert changes in capacitance to voltage (or current); these circuits can be easily found in literature and are well known to those skilled in the art of electronic circuit design. Commercial integrated circuits (or chips) designed for capacitance detection are also available. An example of the circuit designs for capacitance detection is disclosed in U.S. Pat. No. 6,731,121, “Highly Configurable Capacitive Transducer Interface Circuit”, to Hsu et al, the entirety of which is fully incorporated by reference herein.
Virtually all currently manufactured micro gyros produce output signals in which the amplitude is proportional to angular rate. In other words, most micro gyros act as transducers that modulate a carrier signal's amplitude in proportion to angular rate. Signal transmission using amplitude modulation is common and is particularly well known for the Amplitude Modulated or AM radio transmission. Although widely used, amplitude modulation has significant limitations. Even with the best filtering methods (such as Phase Lock Loop), many sources of noise still pass through the filter; thus, many leading micro gyros yield only moderate performance as compared to the optical or mechanical gyros. The wide range of noise sources include drive signal cross-talk, quadrature error, parasitic capacitance, circuit noise, voltage source noise, as well as electrical signal anomalies due to manufacturing imperfection, environmental and packaging stresses.
An alternate signal transmission technique that performs superior to amplitude modulation is the frequency modulation. This approach produces an output signal with a carrier frequency that shifts proportionally to the input rate. Frequency modulation can significantly improve micro gyro performance due to it superior noise suppression capability. Although it is possible to design circuits that digitize an amplitude modulated signal and convert it into a frequency modulation, such approach introduces additional noise in the conversion process. The ideal micro gyro design is a transducer element that intrinsically produces an output signal whose frequency shifts proportionally to the Coriolis force. A wide range of circuit designs are commonly used for demodulating frequency modulated signals and are well known to those of ordinary skill in the art of signal transmission circuit design. For micro gyro applications, what is lacking to take advantage of the frequency modulation is the gyro transducer element. An intrinsically frequency modulated micro gyro transducer is the motivation for this invention.
A new design approach and fabrication solution is needed to achieve the higher performance of frequency modulated micro gyros. The claimed invention overcomes the above discussed limitations of the amplitude modulated micro gyros, thus yielding higher resolution, while retaining the benefits of small size, robust structure, and ease of fabrication.